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Make iter::Empty<T> Send and Sync for any T #68348

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merged 2 commits into from
Jan 19, 2020

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KamilaBorowska
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@KamilaBorowska KamilaBorowska commented Jan 18, 2020

Continuing from #57682

It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of iter::empty", but then a year later I got a compilation error because it wasn't Send and Sync.

Unfortunately, PhantomData<fn() -> T> still errors out. Oh well. I proposed struct PhantomFnWorkaround<T>(fn() -> T);, but dtolnay did not like it, so using explicit implementations.

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r? @pietroalbini

(rust_highfive has picked a reviewer for you, use r? to override)

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 18, 2020
@Centril Centril added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. relnotes Marks issues that should be documented in the release notes of the next release. labels Jan 18, 2020
@Centril Centril added this to the 1.42 milestone Jan 18, 2020
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Centril commented Jan 18, 2020

r? @dtolnay

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nagisa commented Jan 19, 2020

@bors r+

This makes perfect sense and has no future compat concerns, so an obvious r+.

FWIW this PR could also be extended to other auto-traits such as Unpin too.

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bors commented Jan 19, 2020

📌 Commit d7a18f8 has been approved by nagisa

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 19, 2020
Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this pull request Jan 19, 2020
Make iter::Empty<T> Send and Sync for any T

Continuing from rust-lang#57682

It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`.

Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed `
struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.
bors added a commit that referenced this pull request Jan 19, 2020
Rollup of 5 pull requests

Successful merges:

 - #67682 ([const-prop] Remove useless typedef)
 - #68133 (Slimmer syntax)
 - #68154 (Add more BTreeMap/BTreeSet benchmarks regarding iteration)
 - #68247 (Clean up err codes)
 - #68348 (Make iter::Empty<T> Send and Sync for any T)

Failed merges:

 - #68353 (Remove `rustc_error_codes` deps except in `rustc_driver`)

r? @ghost
Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this pull request Jan 19, 2020
Make iter::Empty<T> Send and Sync for any T

Continuing from rust-lang#57682

It's quite funny, when I initially submitted this pull request, I said "Likely nobody will be using that property of `iter::empty`", but then a year later I got a compilation error because it wasn't `Send` and `Sync`.

Unfortunately, `PhantomData<fn() -> T>` still errors out. Oh well. I proposed `
struct PhantomFnWorkaround<T>(fn() -> T);`, but dtolnay did not like it, so using explicit implementations.
bors added a commit that referenced this pull request Jan 19, 2020
Rollup of 3 pull requests

Successful merges:

 - #67682 ([const-prop] Remove useless typedef)
 - #68247 (Clean up err codes)
 - #68348 (Make iter::Empty<T> Send and Sync for any T)

Failed merges:

r? @ghost
@bors bors merged commit d7a18f8 into rust-lang:master Jan 19, 2020
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request Mar 13, 2020
Version 1.42.0 (2020-03-12)
==========================

Language
--------
- [You can now use the slice pattern syntax with subslices.][67712] e.g.
  ```rust
  fn foo(words: &[&str]) {
      match words {
          ["Hello", "World", "!", ..] => println!("Hello World!"),
          ["Foo", "Bar", ..] => println!("Baz"),
          rest => println!("{:?}", rest),
      }
  }
  ```
- [You can now use `#[repr(transparent)]` on univariant `enum`s.][68122] Meaning
  that you can create an enum that has the exact layout and ABI of the type
  it contains.
- [There are some *syntax-only* changes:][67131]
   - `default` is syntactically allowed before items in `trait` definitions.
   - Items in `impl`s (i.e. `const`s, `type`s, and `fn`s) may syntactically
     leave out their bodies in favor of `;`.
   - Bounds on associated types in `impl`s are now syntactically allowed
     (e.g. `type Foo: Ord;`).
   - `...` (the C-variadic type) may occur syntactically directly as the type of
      any function parameter.

  These are still rejected *semantically*, so you will likely receive an error
  but these changes can be seen and parsed by procedural macros and
  conditional compilation.

Compiler
--------
- [Added tier 2* support for `armv7a-none-eabi`.][68253]
- [Added tier 2 support for `riscv64gc-unknown-linux-gnu`.][68339]
- [`Option::{expect,unwrap}` and
   `Result::{expect, expect_err, unwrap, unwrap_err}` now produce panic messages
   pointing to the location where they were called, rather than
   `core`'s internals. ][67887]

* Refer to Rust's [platform support page][forge-platform-support] for more
information on Rust's tiered platform support.

Libraries
---------
- [`iter::Empty<T>` now implements `Send` and `Sync` for any `T`.][68348]
- [`Pin::{map_unchecked, map_unchecked_mut}` no longer require the return type
   to implement `Sized`.][67935]
- [`io::Cursor` now derives `PartialEq` and `Eq`.][67233]
- [`Layout::new` is now `const`.][66254]
- [Added Standard Library support for `riscv64gc-unknown-linux-gnu`.][66899]


Stabilized APIs
---------------
- [`CondVar::wait_while`]
- [`CondVar::wait_timeout_while`]
- [`DebugMap::key`]
- [`DebugMap::value`]
- [`ManuallyDrop::take`]
- [`matches!`]
- [`ptr::slice_from_raw_parts_mut`]
- [`ptr::slice_from_raw_parts`]

Cargo
-----
- [You no longer need to include `extern crate proc_macro;` to be able to
  `use proc_macro;` in the `2018` edition.][cargo/7700]

Compatibility Notes
-------------------
- [`Error::description` has been deprecated, and its use will now produce a
  warning.][66919] It's recommended to use `Display`/`to_string` instead.
- [`use $crate;` inside macros is now a hard error.][37390] The compiler
  emitted forward compatibility warnings since Rust 1.14.0.
- [As previously announced, this release reduces the level of support for
  32-bit Apple targets to tier 3.][apple-32bit-drop]. This means that the
  source code is still available to build, but the targets are no longer tested
  and no release binary is distributed by the Rust project. Please refer to the
  linked blog post for more information.

[37390]: rust-lang/rust#37390
[68253]: rust-lang/rust#68253
[68348]: rust-lang/rust#68348
[67935]: rust-lang/rust#67935
[68339]: rust-lang/rust#68339
[68122]: rust-lang/rust#68122
[67712]: rust-lang/rust#67712
[67887]: rust-lang/rust#67887
[67131]: rust-lang/rust#67131
[67233]: rust-lang/rust#67233
[66899]: rust-lang/rust#66899
[66919]: rust-lang/rust#66919
[66254]: rust-lang/rust#66254
[cargo/7700]: rust-lang/cargo#7700
[`DebugMap::key`]: https://doc.rust-lang.org/stable/std/fmt/struct.DebugMap.html#method.key
[`DebugMap::value`]: https://doc.rust-lang.org/stable/std/fmt/struct.DebugMap.html#method.value
[`ManuallyDrop::take`]: https://doc.rust-lang.org/stable/std/mem/struct.ManuallyDrop.html#method.take
[`matches!`]: https://doc.rust-lang.org/stable/std/macro.matches.html
[`ptr::slice_from_raw_parts_mut`]: https://doc.rust-lang.org/stable/std/ptr/fn.slice_from_raw_parts_mut.html
[`ptr::slice_from_raw_parts`]: https://doc.rust-lang.org/stable/std/ptr/fn.slice_from_raw_parts.html
[`CondVar::wait_while`]: https://doc.rust-lang.org/stable/std/sync/struct.Condvar.html#method.wait_while
[`CondVar::wait_timeout_while`]: https://doc.rust-lang.org/stable/std/sync/struct.Condvar.html#method.wait_timeout_while
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